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On A Large Array Of Midsized Telescopes

On A Large Array Of Midsized Telescopes. Stephen Fegan Vladimir Vassiliev UCLA. Primary Science Goals. Detect and measure VHE transients at cosmological distances Self triggering Measure lightcurve from Mrk-421 at z=1 with few minute resolution Survey of VHE sky to level of 1-2 mCrab

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On A Large Array Of Midsized Telescopes

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  1. On A Large Array Of Midsized Telescopes Stephen FeganVladimir VassilievUCLA

  2. Primary Science Goals • Detect and measure VHE transients at cosmological distances • Self triggering • Measure lightcurve from Mrk-421 at z=1 with few minute resolution • Survey of VHE sky to level of 1-2 mCrab • Detailed observations of Galactic sources in the energy range 20GeV to >50 TeV On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  3. Requirements From Science Goals • To resolve a few min variability time scale in the emission of aMrk 421-like AGN at z=1 the collecting area must be ~1km2. • To survey the sky to 1-2 mCrab over a few years of operation, must have VERITAS sensitivity over full sky • 1km2 collecting area → Crab Nebula rate of ○1g/min >10 TeV ○ 2g/hr >100 TeV E Interval Rate[GeV] [min`1]25-50 1.350-100 0.7100-200 0.3 On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  4. Baseline Design • Array • 217 telescopes • 8 hexagonal rings + 1 • 80m separation • Telescope and Detector • ø10m equivalent • QE = 0.25 (Bialkali) • 15º field of view • Facts and Figures • Outer radius: 640m • Single cell area: 5543m2 • Total area: 1.06km2 Distance From Center Of Array [m] Distance From Center Of Array [m] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  5. Collecting Area vs. Field Of View Current IACTAs Narrow field of view <0.01 km2 @ 40 GeV 0.05-0.1 km2 @ 100 GeV 0.2-0.3 km2 @ 10 TeV Field of view [πsr] Field of view [deg] Square KM Array Continuum of modes Trade area for solid angle Parallel mode Narrow field of view 1 km2 @ 40 GeV 2 km2 @ 100 GeV 4-5 km2 @ 10 TeV “Fly’s Eye” mode Wide field of view 0.02-0.03 km2 @ 40 GeV 0.1-0.2 km2 @ 100 GeV 3-4 km2 @ 10 TeV Collecting Area [km2] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  6. All Sky Coverage: “Fly’s Eye” Mode Each telescope points in different direction. If position of telescope n on ground is (xn,yn) Zenith = ξ × ( xn2 + yn2 )1/2 Azimuth = tan-1( xn / yn ) On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  7. Question(s) How can an array of mid-sized telescopes operate in the E~30 GeV range? OR Why is the collecting area of instruments like VERITAS so large at E>100GeV? On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  8. Cell Effect – Collecting Area Gamma-rays INSIDE detector Instrument has efficiency ε(E) such that effective area is: AI=ε(E)πRI2 Gamma-rays OUTSIDE detector Instrument detects γ’s to radius RO(E) such that effective area is: AO=ε(E)πRO(E)(2RI+RO(E)) Energy Dependence ε(E): 0.4 @ 20 GeV 0.8 @ 40 GeV RO(E): <80m @ 20 GeV 600m @ 10 TeV OUTSIDE DETECTOR INSIDEDETECTOR RO RI On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  9. Cell Effect – Small Impact Parameter • Infinite Array Of Telescopes • 3500m ASL→RCherenk = 85m • DScopes = 80m Distance [m] • Geometry Dictates That • Impact point of every shower is in some cell • BMax = 47m • At least 3 telescopes contained in Cherenkov light pool Distance [m] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  10. Cell Effect – Cherenkov PE Density • PE density after: • Atmosphere • Mirror reflection • Photocathode Photoelectron density [PE/m2] Cell Geometry Consider only the density within 80m of core Midsized telescopes ø10m, A=78m2 E=32 GeV, b=80m →nPE=78 Distance from shower core [m] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  11. Trigger – Threshold vs. Pixel Size Normalized trigger threshold for given QE, NSB rate and FOV QE=0.25, FoV=15o, Rnsb=0.1 kHz QE=0.25, FoV=10o, Rnsb=0.1 kHz (nth-Nnsb) /Q QE=0.25, FoV=15o, Rnsb=1.0 kHz QE=0.25, FoV=10o, Rnsb=1.0 kHz Importance of effect: 1) QE 2) Rate 3) FoV QE=0.5, FoV=15o, Rnsb=0.1 kHz QE=0.5, FoV=10o, Rnsb=0.1 kHz QE=0.5, FoV=15o, Rnsb=1.0 kHz QE=0.5, FoV=10o, Rnsb=1.0 kHz Trigger Pixel Size [degree] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  12. Trigger – Efficiency vs Pixel Size – I (Central Telescope) Parameters: Eg=42 GeV FoV=15o Rnsb=1kHz QE: 1.0, D=7m QE: 0.5, D=10m El: 3.5 km QE: 0.5, D=7m QE: 0.25, D=10m Optimum trigger sensor pixel size is 0.07o-0.3o Central Telescope Trigger Efficiency Weak dependence on QE, D, El Trigger Pixel Size [degree] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  13. Trigger – Efficiency vs Pixel Size – II (Full Array) Array Trigger: Three telescopes above operational threshold p=0.05o p=0.08o p=0.10o Array Trigger Efficiency p=0.13o p=0.16o Array Parameters: Elevation: 3.5 km QE: 0.25 Reflector: 10 m FoV: 15o p=0.20o Photon Energy [GeV] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  14. Trigger – Peak Detection Energy Diff. spectral index: 2.5 12 GeV Diff. Rate 15 GeV Trigger Efficiency 20 GeV El=4.5km, QE: 1.0, D=7m El=4.5km, QE: 0.5, D=10m El=3.5km, QE: 1.0, D=7m El=3.5km, QE: 0.5, D=10m El=4.5km, QE: 0.5, D=7m El=4.5km, QE: 0.25, D=10m El=3.5km, QE: 0.5, D=7m El=3.5km, QE: 0.25, D=10m 27 GeV Photon Energy [GeV] Photon Energy [GeV] Effects: 1) “Cell operation” mode 2) Optimum trigger pixel size 3) QE, Reflector Size 4) Elevation 5) Rnsb Parameters: Trigger pixel size: 0.146o Un-localized source (FoV=15o) Rnsb: 1kHz On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  15. El=4.5km, QE: 1.0, D=7m El=4.5km, QE: 0.5, D=10m El=3.5km, QE: 1.0, D=7m El=3.5km, QE: 0.5, D=10m QE: 0.5 El: 4.5 km, D=7m QE: 0.25 El: 4.5 km, D=10m QE: 0.5 El: 3.5 km, D=7m QE: 0.25 El: 3.5 km, D=10m Trigger – Telescope Multiplicity Average Number of Telescopes in Trigger 30 GeV g triggers 5 telescopes Photon Energy [GeV] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  16. Cleaning – Sample Event Photon direction [deg] Photon direction [deg] Photon direction [deg] Event 1 (42 GeV) Event 2 (42 GeV) On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  17. Cleaning – Voronoi Diagram 0.0 0.3 -0.1 0.2 -0.2 0.1 Photon direction [deg] -0.3 0.0 -0.4 -0.1 -0.5 -0.2 -0.6 -0.3 -0.7 -0.4 -0.8 -0.5 -0.9 -0.6 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 -0.1 0.0 0.1 0.2 0.3 0.5 0.5 0.6 0.7 0.8 Photon direction [deg] Photon direction [deg] Event 1 (42 GeV) Event 2 (42 GeV) On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  18. Cleaning – P.E. Separation Scales g: 21 GeV NSB: 150 g/deg2 g: 42 GeV NSB: 150 g/deg2 Diff. density [Arbitrary] P.E. separation [deg] g: 100 GeV NSB: 150 g/deg2 QE: 0.25 Reflector Diameter: 10m Elevation: 3.5 km Trigger pixel size: 0.146o Voronoi Diagram P.E.-P.E. separation scales in Image: 0.015o-0.045o On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  19. Reconstruction – Angular Acceptance Optimum cut: 4 photons within circle of 0.02o radius 21 GeV 42 GeV 100 GeV CR Event containment fraction [1] q radius [deg] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  20. Cleaning – Sample Event Single 42 GeV event View from 4 telescopes Optimal cleaning (from consideration of angular reconstruction) keeps only photons near core Multiple cleaning schemes may be appropriate. ● Shower axis ● Shape cut ● Energy estimate On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  21. Background Rejection – Shape Cut Parameter <d> gives the width of the emission region in space 21 GeV 42 GeV 100 GeV CR NPE [1] Electromagnetic showers: tightly confined along shower axis, have small <d> Hadronic events: reconstructed cascade radius is larger than for gamma-rays Mean cascade radius <d> [m] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  22. Background Rejection – Shower Max Parameter <p> gives the mean emission height of the Cherenkov photons. 21 GeV 42 GeV 100 GeV CR q < 0.2o No q cut Ln(Ng) [1] 1st interaction Proton: 70g/cm2 Gamma: 37g/cm2 Distribution of reconstructed <p> different for each species. Mean emission height above array [m] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  23. 21 GeV 42 GeV 100 GeV CR 21 GeV 42 GeV 100 GeV Integrated flux [p/deg2/min] Integrated flux [p/deg2/min] Mean emission height above array [m] g containment [fraction] 21 GeV 42 GeV 100 GeV 21 GeV 42 GeV 100 GeV S/N=S/SRT(B) [arbitrary] S/N=S/SRT(B) [arbitrary] Mean emission height above array [m] g containment [fraction] Background Rejection – Shower Max On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  24. Lessons • “Cell Effect”: operation in 20-30 GeV range possible with midsized telescopes • Optimum trigger pixel size is ~0.1º • Optimum image pixel size is ~0.01º • Very “hard” cleaning required to optimize reconstruction of shower axis • Multiple cleaning regimes is suggested • Reconstructed emission height can be used to reject protons On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  25. Comparison – Collecting Area Collecting Area [m2] Energy [GeV] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  26. Comparison – Differential Rate Differential Rate [arbitrary] Energy [GeV] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

  27. All Sky Survey – One Year Sensitivity Sensitivity [mCrab] On A Large Array Of Midsized Telescopes Fegan & Vassiliev, UCLA

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